Refactor MD5 code. Add unit test for FileFinder.

Summary: FileFinder::find() now uses PHP's md5_file() for all MD5 calculations and now has associated unit tests.

Test Plan: Ensure the unit tests pass.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5013